Steam generating apparatuses are used for heating water in order to generate steam which can be used for example for ironing cloth. In these apparatuses the steam is generated within a boiler in which the temperature of the water can be controlled within a certain temperature range by means of a heating device and a temperature sensor as follows: When the temperature signal of the temperature sensor indicates, that the temperature of the water falls below a certain level, the heating device is activated and the water is heated. If the temperature signal indicates, that the water temperature rises above a certain level, the heating device is deactivated.
Generally, in such steam generating apparatuses, the boiler is refilled with water from a larger water tank automatically. As a prerequisite for realizing a smooth refilling operation, the water level inside the boiler has to be detected, such that water is refilled into the boiler when the water level drops below a certain water level. A water level sensor which would be arranged inside the boiler would very soon provide inaccurate results due to calcification of the inside of the boiler or the usage of de-mineralized water which can cause inaccurate sensing.
EP 0 843 039 B1 describes a steam generator which comprises an evaporation boiler associated with heater elements and fed with water by a pump, a solenoid valve for expelling steam, and regulator means including a temperature sensor or a pressure sensor for monitoring the quantity of water introduced into the boiler and for controlling the pump, wherein the regulator means are associated with an electronic system for taking successive measurements for actuating the pump as a function of the value of the slope of a curve representing variation in temperature or pressure, said slope being analyzed by the electronic system.
However, actuating the pump according to the value of the slope of the curve representing variation in temperature requires a very accurate and expensive temperature sensor since this slope method requires a very direct heat transfer through a thin wall such that slightest variations of the slope of the curve are detected. Also the mounting of such a temperature sensor costs much effort because the required sensing accuracy requires a very good heat conductivity between the temperature sensor and the water inside the boiler. The required sensing accuracy requires arranging the temperature sensor at a flat portion for a proper mounting of the sensor, which in turn complicates the forming of the shell. A heat conductive paste may be applied between the temperature sensor and the boiler shell. However, this makes additional mounting processes necessary. Arranging the temperature sensor somewhere else is not possible since the sensing accuracy would not be sufficient for determining the water level with the method described in EP 0 843 039 B1.
It is therefore an object of the invention to provide an alternative method of determining the liquid level in a boiler of a steam generating apparatus providing a comparable accuracy in determining the liquid level and providing more flexibility in arranging a required temperature sensor.